Improve with_source_map.

Rename `with_source_map` as `set_source_map`. Because `with` functions
(e.g. `with_session_globals`, `scoped_tls::ScopedKey::with`) are for
*getting* a value for the duration of a closure, and `set` functions
(e.g. `set_session_globals_then` `scoped_tls::ScopedKey::with`) are for
*setting* a value for the duration of a closure.

Also fix up the comment, which is wrong:
- The bit about `TyCtxt` is wrong.
- `span_debug1` doesn't exist any more.
- There's only one level of fallback, not two.

(This is effectively a follow-up to the changes in #93936.)

Also add a comment explaining that `SessionGlobals::source_map` should
only be used when absolutely necessary.

compiler/rustc_interface/src/interface.rs

@@ -292,7 +292,7 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
                 override_queries: config.override_queries,
             };
 
-            rustc_span::with_source_map(compiler.sess.parse_sess.clone_source_map(), move || {
+            rustc_span::set_source_map(compiler.sess.parse_sess.clone_source_map(), move || {
                 let r = {
                     let _sess_abort_error = OnDrop(|| {
                         compiler.sess.finish_diagnostics(registry);